A heart-lung machine is typically used during heart surgery for coronary artery bypass, valvular replacement or proximal aortic reconstruction. Such a machine substitutes for the function of the heart muscle to pump blood throughout the body, and substitutes for lung function by removing carbon dioxide and adding oxygen to the patient's blood.
To use the heart-lung machine, full arrest of the coagulation cascade in the blood is required to prevent clot formation on the surfaces of the machine. In the coagulation cascade fibrin formation is initiated with the release from damaged cells of a complex protein known as tissue factor. This initiates the coagulation cascade which ultimately results in fibrin generation. The arrest of the cascade is accomplished typically by administering heparin to the patent. Heparin impedes coagulation by enhancing the effectiveness of anti-thrombin III, a naturally occurring inhibitor of coagulation. It does this by causing a conformational change that exposes additional binding sites on the anti-thrombin III molecule, which increases the ability of antithrombin III to bind with factors XIIa, XIa, IXa and Xa, which in turn accelerates their ability to inhibit the formation of fibrin. After the period of cardiopulmonary bypass is completed, the heparin effect is reversed by administering an antagonist agent, such as protamine.
Determining the proper number of units of heparin to be administered is not easily determined because of two phenomena. First, the amount of heparin that must be injected to achieve a certain plasma heparin concentration varies from patient to patient. Second, a given heparin level does not reflect an exact state of anticoagulation in a particular patient because of a number of factors peculiar to individual patients such as extravascular depots, hemodilution, hypothermia, heparin resistance and anti-thrombin III deficiency.
To determine whether the heparin administered has effectively reduced the ability of the blood to clot, the activated coagulation time (ACT) is measured. The ACT was introduced by Hattersley in 1966 and is a method for the rapid determination of the Lee-White whole blood clotting time. Although initially performed by manual rotation of a test tube and visual inspection for the presence of a clot, the test is typically performed via an automated method known as the HEMOCHRON (International Technidyne, Edison, N.J.). Typically, a sample containing two cc of whole blood is obtained and placed in an ACT tube and the time recorded. The tube is shaken to mix the blood with a diatomaceous powder which promotes coagulation by its high surface area. The tube is then warmed to 37.degree. C. A magnetic rod placed in the tube is observed by a magnetic detector, and when coagulation occurs, the rod is displaced, signalling completion of the test. This test typically takes about 107.+-.13 seconds in a patient with normal coagulation status.
The ACT is currently first measured to provide a baseline ACT and then is measured again after administration of heparin to document whether a safe level of anticoagulation has been attained. The ACT is also measured serially during surgery, usually about every 30 minutes, to be sure that adequate anticoagulation is maintained in the face of metabolism changes and excretion of heparin.
The ACT is also used after surgery is completed. At this time, the heart has been restarted and is pumping blood through the lungs where oxygen is added to the blood and carbon dioxide is removed. The heparin anticoagulation effect is reversed by the administration of an antagonist to heparin, such as protamine. Protamine is polycationic and forms a complex with heparin, thus reversing its effects on Anti-Thrombin III. After administering protamine, the ACT is measured to determine if the protamine has adequately reversed the effects of heparin.
Sometimes, however, the administration of protamine does not return the ACT to the baseline (normal) condition and the patient may experience bleeding. Although many clinicians associate an increased ACT with a prolonged heparin effect, the ACT is limited in that it is a test of essentially the entire coagulation system, and as such, it is affected by other changes in the coagulation cascade. Therefore, an elevated ACT after heparin reversal with protamine does not necessarily indicate that residual heparin is the cause of the elevated ACT. In addition to residual heparin, destruction of serine protease (proteins required for blood to clot, otherwise known as clotting factors) hypofibrinogenemia, fibrinolysis and platelet abnormalities, both qualitative and quantitative, can influence the ACT. Decreased levels of fibronectin, a substance which is involved in platelet adhesion, may also result in increased bleeding.
One device currently used to assist in determining protamine dose is the HEPCON (HemoTec Inc., Englewood, Colo.). The HEPCON device consists of four chambers which contain specific amounts of protamine, thromboplastin and diluent. Air bubbles percolate through a blood sample in each chamber until a photocell detects clot formation in one of the chambers. Based upon the patient's height and weight, the device computes the heparin level. In essence, this device confirms whether or not a patient's bleeding tendency is due to heparin. If protamine administration is followed by obtaining an elevated ACT and a HEPCON test produces a reading of zero, this indicates that no heparin is circulating and it is likely that one or more other etiologies may be responsible for the hemorrhaging.
Bleeding in general surgical or non-surgical patients, including those involved in cardiopulmonary bypass surgery where there is no longer any circulating heparin, could be due to a decreased level of coagulation factors such as factors V, VIII, XIII and fibrinogen, as well as thrombocytopenia, or more commonly, abnormal platelet function, decreased levels of fibronectin, complement activation and fibrinolysis. Decreased levels of serine proteases and platelets could be due to low grade coagulation during bypass with consumption of the factors and platelets, or more likely, damage and destruction sustained when exposed to the surface of the oxygenator.
The anesthesiologist and surgeon are often faced with the situation that a patient is bleeding significantly and it is not due to heparin. A similar situation may occur in patients with massive bleeding due to a medical etiology. Because of the numerous possibilities of which factor or combination of factors is needed to stem the hemorrhaging, combined with the extremely limited amount of time available, the patient is frequently treated with a shotgun therapy, for example, by administration of platelets, fresh frozen plasma, desmopressin acetate (DDAVP) and sometimes epsilo-amino caproic acid (AMICAR) and cryoprecipitate. Since the use of platelets, fresh frozen plasma and cryoprecipitate all carry the risk of disease transmission, a system to rapidly determine if one or two therapies would be sufficient would decrease the risk to the patient of contracting hepatitis, AIDS, and numerous other diseases. Furthermore, in cases where it is determined that DDAVP or AMICAR would be therapeutic, the patient would be spared transfusion of blood products altogether. An additional reason to rapidly determine the specific appropriate therapy is that as long as there is a deficiency in blood coagulation, the patient will require transfusion of packed red blood cells (PRBCs). In addition to the risk of disease transmission, transfusion of large amounts of PRBCs dilutes the coagulation factors and platelets in the blood, resulting in a "dilutional coagulopathy", thus possibly worsening the degree of hemorrhaging.
At present, complete, definitive coagulation studies can only be done in the laboratory, which takes too long to be of use in determining a specific therapy against massive hemorrhage associated with an elevated ACT, whether under operating room or non-operating room conditions. -There is a need for a method that is rapid enough to allow a doctor to determine and administer a specific therapy under the severe time constraints posed by an episode of rapid massive bleeding whether in the operating room or otherwise.